Thin magnetisable layers are currently used on an industrial scale mainly for the purpose of magnetic data storage. Since they are in general not self-supporting, they are applied to a suitable layer carrier. During this-stage of manufacture, the magnetic properties of the thin layers must be measured by way of a quality control.
Depending on the method of use and the physical principle of the recording and reproduction process, it may be required, in extreme cases, that the complete hysteresis loop must be determined. Occasionally it is sufficient to know the characteristic values of the hysteresis loop: Coercive field strength Ho, residual magnetism Mat, and squareness S'.
One of the most frequently-used methods of measurement is based on the exploitation of the induction law (Faraday's Law): EQU U=cdF/dt
where U=induced electrical voltage, c=const., and F=magnetic flux. In this situation, in the first instance, a part surface of the layer is magnetised by means of a variable external magnetic field H. This magnetisation is achieved in most cases by what is referred to as a magnetic head, which, depending on the application, allows for a variable magnetic field strength H to be created either perpendicular to or parallel to the layer.
The strength of the magnetic induction field B deriving from the magnetised layer, and therefore the magnetisation M, are measured in turn by means of a magnetic head. Its gap aperture is oriented in such a way that it forms the lowest possible resistance for the magnetic lines of force, which in practical terms are short-circuited and bundled in the magnetic head. The flux is sensed in this context by an induction coil. The value of the induced voltage then depends on the strength of the change in the flux.
An arrangement such as this is described in IBM TDB, Vol. 24, page 6209 f., April 1982. It features erasing, recording, and reading heads, which are mounted on a circular plate, which can be moved by a motor vertical to the plane of the magnetic plate which is to be investigated. During the test, the magnetic plate remains in a position of rest, and the magnetic heads are stroked across the specimen by the plates being set in rotation. A number of sections from the magnetic plate are tested, by means of the plate being moved further in a linear direction by a slide element.
The disadvantage with such an inductive (indirect), non-static measurement is that a high relative speed of test specimen and measuring head is required for a precision measurement, and that considerable damages result from an accident or from their coming in contact. Due to the proportional ratio between the induced voltage and the magnetic flux, and due to the relationship F=c.intg.dA; where c=const., B=magnetic induction, and dA=infinitesimal surface element, a dependency is derived between the measured signal and the magnetised surface.
In addition to this, a signal is obtained only at points of change of flux. Accordingly, this method is not very sensitive in relation to the value which is actually of interest, namely the magnetisation of the surface as a whole. In view of the fact that, as the recording field strength increases, so the transition area between "recorded" and "unrecorded" is "smeared" by feedover, and, in the case of a conductive substrate, by eddy currents, and the signal also has to be integrated, then in principle measuring errors can arise with this method of measurement.
It is therefore the task of the present invention to create a process and a device for the determination of the magnetic parameters of thin layers, in which the disadvantages outlined above can be avoided.